Hermetic compressor

ABSTRACT

In a hermetic-type compressor, a piston has a sliding face which slides on an inner wall of the cylindrical hole and is reciprocatably inserted in the cylindrical hole. A connecting rod connects an eccentric shaft part and the piston. The cylindrical hole has a tapered part, whose inside diameter dimension gradually increases from a top dead center of the piston toward a bottom dead center, and an end part on the shaft side. A reciprocation direction of the piston is substantially a horizontal direction. A recessed part, which is recessed to the inside in a radial direction of the piston and holds lubricating oil, is provided in the sliding face of the piston. A part on the lower side in the vertical direction of the piston which comes into contact with the end part on the shaft side of the cylindrical hole when the piston is positioned in the bottom dead center is a part of the sliding face.

TECHNICAL FIELD

The present invention relates to a hermetic-type compressor for use in a refrigeration cycle system such as a refrigerator freezer.

BACKGROUND ART

A reciprocation-type hermetic compressor has, as a compression mechanism, a cylinder forming a cylindrical compression chamber, a cylindrical piston, and a connecting rod. The piston reciprocates in the cylinder. By the connecting rod, an eccentric shaft of a shaft is connected to the piston via a piston pin. The shaft is fixed to the shaft center of a rotor of a motor, and the compression mechanism is operated by the rotation of the rotor.

In such a hermetic-type compressor, a gap is necessary between the inner peripheral face of the cylinder and a sliding face of the piston so that the faces slide each other. However, when the gap is large, a blowby gas as a leaked high-temperature high-pressure refrigerant gas compressed in the compression chamber is generated, and the compression efficiency deteriorates. On the other hand, when the gap is small, a sliding loss increases, and input-output efficiency deteriorates.

Consequently, a hermetic-type compressor using a cylinder formed so that the inside diameter dimension of a compression chamber is gradually increased from a side on which a piston is positioned in a top dead center toward a side on which the piston is positioned in a bottom dead center has been proposed (refer to, for example, patent literature 1). FIGS. 16A and 16B are cross sections of a compression part of a conventional hermetic-type compressor described in the patent literature 1. FIG. 16A shows a state where the piston is positioned in the bottom dead center, and FIG. 16B shows a state where the piston is positioned in the top dead center.

Cylinder block 14 includes cylinder 16 having a center axis in an almost horizontal direction. Piston 23 inserted in an almost horizontal direction is connected to connecting rod 26 via a piston pin (not shown), thereby constructing piston assembly 23A. At an end face (an end face on the right side in the drawing) of cylinder 16 on the side opposite to connecting rod 26, a valve plate (not shown) is attached. By piston 23, cylinder 16, and the valve plate constructed as described above, compression chamber 15 is formed. Piston 23 reciprocates in an almost horizontal direction in cylinder 16 via connecting rod 26 by eccentric motion of an eccentric shaft (not shown) of a shaft (not shown).

The inner face of cylinder 16 is formed so as to have tapered part 17 whose inner diameter dimension increases from Dt to Db (>Dt) from a some midpoint on the side where piston 23 is positioned in the top dead center toward the side where piston 23 is positioned in the bottom dead center. Piston 23 is formed so that its outer diameter dimension is almost the same in full length. Consequently, around the top dead center where the pressure in compression chamber 15 is high, the gap in the sealing part of piston 23 is reduced, and blow-by gas is prevented. On the other hand, around the bottom dead center, the gap increases, so that sliding loss can be reduced.

However, piston 23 constructed as described above repeats reciprocating while always slightly vibrating in all directions in the gap with the inner face of cylinder 16 for the following reason. At the time of operation, a dynamic compressive load, the inertia force and gravity of movable members such as piston 23 and connecting rod 26, and a piston lateral pressure load generated by converting the rotational motion to the reciprocating motion act on piston 23. Forces such as sliding resistance of the sliding part exert influences one another, and act on piston 23 while the directions and magnitudes of the forces are changing. Such an action is also a factor of slight vibrations in all directions of piston in the gap with the inner face of cylinder 16.

Particularly, in a state where piston 23 is positioned around the bottom dead center, the gap with tapered part 17 of cylinder 16 becomes larger than the gap around the top dead center. Since the center axis of cylinder 16 is disposed in an almost horizontal direction, by the influence of the gravity of piston assembly 23A, the bottom dead center side of piston 23 leans more vertically downward. As a result, connecting rod 26 side of piston 23 leans more vertically downward.

Due to occurrence of the slight vibration behavior by the reciprocating motion of piston 23 and pressure applied to piston 23, the sliding part of piston 23 and tapered part 17 of cylinder 16 locally slide each other. There is the possibility that such a local sliding generates a contact sound and causes abrasion starting from the contact part.

The structure that entire piston 23 is disposed in cylinder 16 when piston 23 is in the bottom dead center position relatively improves stability of the behavior in tapered part 17 of cylinder 16. However, in the structure, the total length of cylinder 16 is long, and the size of the compression mechanism is inevitably large. Accordingly, the entire hermetic-type compressor becomes large. As a result, it is difficult to reduce the weight, and it is accordingly difficult to save resources.

CITATION LIST Patent Literature

PTL 1: Unexamined Japanese Patent Publication No. 2002-89450

SUMMARY OF THE INVENTION

The present invention relates to a hermetic-type compressor realizing prevention of noise and improved efficiency and reliability by avoiding a local contact between a piston and an inner face of a cylinder (cylindrical hole), simultaneously, by minimizing sliding area, and preventing generation of noise due to a contact between the piston and the cylinder and a local contact causing abrasion.

A hermetic-type compressor of the present invention has a sealed container, an electric mechanism, and a compression mechanism. The sealed container stores lubricant oil at its bottom. The electric mechanism and the compression mechanism are disposed in the sealed container. The electric mechanism drives a compression mechanism. The compression mechanism includes a shaft, a cylinder block, a piston, and a connecting rod. The shaft has a main shaft part rotated by the electric mechanism and an eccentric shaft part formed in the main shaft part. The cylinder block has a cylindrical hole constructing a compression chamber and a bearing rotatably supporting the main shaft part. The cylindrical hole and the bearing are disposed so that axis of the cylindrical hole and that of the bearing are orthogonal to each other. The piston has a sliding face which slides on an inner wall of the cylindrical hole and is reciprocatably inserted in the cylindrical hole. The connecting rod connects the eccentric shaft part and the piston. The cylindrical hole has a tapered part, whose inside diameter dimension gradually increases from a top dead center of the piston toward a bottom dead center, and an end part on the shaft side. The reciprocation direction of the piston is substantially a horizontal direction. A recessed part, which is recessed to an inside in a radial direction of the piston and holds the lubricating oil, is provided in the sliding face of the piston. A part on the lower side in the vertical direction of the piston, which comes into contact with the end part on the shaft side of the cylindrical hole when the piston is positioned in the bottom dead center, is a part of the sliding face.

With the configuration, by the tapered part in the cylindrical hole and the recessed part provided in the piston, the average gap and the sliding area are reduced, and sliding resistance of the piston can be lessened. Around the bottom dead center of the piston, the recessed part in the piston does not come off from the end part on the shaft side of the cylindrical hole. Consequently, the inclination of the piston does not become excessive, and a local collision between the periphery of the recessed part in the piston and the cylinder block can be avoided. Therefore, occurrence of collision sound is suppressed, and increase in noise can be prevented. By holding a large amount of lubricant oil scattered and supplied from the shaft in the recessed part, the lubricant oil can be amply supplied to the gap between the inner face of the cylindrical hole and the surface of the piston. As a result, lubricity and sealing performance between the cylinder and the piston are improved, so that the compression efficiency improves. In addition, the total length of the cylindrical hole is short.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross section of a main part of a hermetic-type compressor prior to a first embodiment of the present invention.

FIG. 2 is a vertical cross section of a main part of another hermetic-type compressor prior to the first embodiment of the present invention.

FIG. 3 is a top view of the main part of the hermetic-type compressor shown in FIG. 2.

FIG. 4 is a cross section showing a state where a piston of a hermetic-type compressor in the first embodiment of the invention is positioned at the bottom dead center.

FIG. 5 is a cross section showing a state where the piston of the hermetic-type compressor illustrated in FIG. 4 is positioned at the top dead center.

FIG. 6 is a bottom view of the piston of the hermetic-type compressor illustrated in FIG. 4.

FIG. 7 is a cross section of a compression part showing a state where the leaned piston of the hermetic-type compressor illustrated in FIG. 4 is positioned at the bottom dead center.

FIG. 8 is a cross section of a compression part showing a state where a piston of a hermetic-type compressor in a second embodiment of the present invention is positioned at the bottom dead center.

FIG. 9 is a cross section showing a state where the piston is positioned at the top dead center, of the compression part illustrated in FIG. 8.

FIG. 10 is a vertical cross section of a piston assembly of the hermetic-type compressor in the second embodiment of the invention.

FIG. 11 is a cross section of a top face part of the compression part showing a state where the piston of the hermetic-type compressor in the second embodiment of the invention is in a compression stroke.

FIG. 12 is a characteristic diagram of the piston lateral pressure load with respect to crank angle of the hermetic-type compressor in the second embodiment of the invention.

FIG. 13 is a characteristic diagram of coefficient of performance with respect to space volume of a recessed part in the hermetic-type compressor in the second embodiment of the invention.

FIG. 14 is a characteristic diagram of the coefficient of performance with respect to distance between recessed parts in the hermetic-type compressor in the second embodiment of the invention.

FIG. 15 is a characteristic diagram of the coefficient of performance with respect to operation frequency of the hermetic-type compressor in the second embodiment of the invention.

FIG. 16A is a vertical cross section of a compression part showing a state where a piston of a conventional hermetic-type compressor is positioned in the bottom dead center.

FIG. 16B is a vertical cross section of the compression part showing a state where the piston illustrated in FIG. 16A is positioned in the top dead center.

DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments.

First Exemplary Embodiment

The inventors of the present invention have proposed another configuration for compression efficiency improvement and reduction in sliding loss (Unexamined Japanese Patent Publication No. 2006-169998). FIG. 1 is a cross section of a main part of a hermetic-type compressor and shows a state where piston 123 is in the bottom dead center.

In the surface of piston 123, narrow circular-shaped grooves 141A and 141B and a recessed part 141C which is recessed to an inside in a radial direction are provided. The inside diameter of cylindrical-shaped hole 116 is almost constant. At a position around the bottom dead center, lower end part 123B of piston 123 and recessed part 141C are exposed from cylindrical-shaped hole 116. Grooves 141A and 141B are partially exposed from notch 114A formed in cylinder block 114. By forming grooves 141A and 141B and recessed part 141C in the sliding face (outer peripheral face) of piston 123 in such a manner, when piston 123 reciprocates, the amount of oil supplied to a sealing part and a sliding part increases. Consequently, the sealing performance improves, and the sliding loss can be reduced while improving the compression efficiency and decreasing the sliding area.

By combining the configuration of FIG. 1 to the configuration of FIG. 16A, further improvement in compression efficiency and reduction in the sliding loss can be expected. FIG. 2 is a vertical cross section of a main part of a hermetic-type compressor on the assumption of the combination. FIG. 2 shows a state where the piston is at the bottom dead center. FIG. 3 is a top view of the main part in a state where the piston of the hermetic-type compressor shown in FIG. 2 is in the compression stroke.

Cylindrical hole 216 has straight part 218 and tapered part 217. In straight part 218, the inside diameter of cylindrical-shaped hole 216 is almost constant. In tapered part 217, the inside diameter dimension increases from Dt to Db (>Dt) from a some midpoint on the side where piston 223 is positioned in the top dead center (the right side in the drawing) toward the side where piston 23 is positioned in the bottom dead center (the left side in the drawing). The gap between piston 223 and tapered part 217 is large around the bottom dead center and small around the top dead center.

In the surface of piston 223, grooves 241A and 241B and recessed part 241C which is recessed to the inner side in a radial direction are formed. In a position around the bottom dead center, lower end part 223B of piston 223 and recessed part 241C are exposed from cylindrical-shaped hole 216. Grooves 241A and 241B are partially exposed from notch 214A formed in cylinder block 214.

Therefore, the sealing part of piston 223 prevents blowby gas by reduction in the gap around the top dead center and the labyrinth seal effect by grooves 241A and 241B. Lubricant oil scattering around the bottom dead center is held in recessed part 241C, and supplied from recessed part 241C to grooves 241A and 241B and the sliding part of piston 223. By increasing the oil supply amount in such a manner, the sealing performance and lubricity can be improved.

As a result, by enlargement of the average clearance of the sliding part of piston 223 and reduction in sliding area, a hermetic-type compressor with largely reduced sliding loss, high sealing performance, and high compression efficiency can be expected. In the configuration, piston 223 is exposed from tapered part 217 of cylindrical-shaped hole 216 around the bottom dead center. At this time, piston 223 has a cantilever configuration that the sliding part of piston 223 inserted in cylindrical-shaped hole 216 serves as a supporting point, and the deadweight of piston 223, a piston pin (not shown), and connecting rod 226 is supported by the supporting point. This is because the clearance of a connection part of connecting rod 226 and an eccentric shaft (not shown) of the crankshaft and the clearance of a connection part of a bearing and the crankshaft (which are not shown) are larger than the gap of the sealing part of piston 223.

Due to the cantilever configuration, at the bottom dead center at which piston 223 is exposed most from cylindrical-shaped hole 216, the bottom dead center side of piston 223 leans vertically downward in the gap formed by tapered part 217 and piston 223. This is because, since it is formed to have tapered part 217 in which the inside diameter dimension of cylindrical-shaped hole 216 increases from Dt to Db, the gap between tapered part 217 and piston 223 increases around the bottom dead center.

If piston 223 does not have recessed part 241C, long support length of the sliding part which supports one side of piston 223 can be assured as shown by L1 in FIG. 2. However, when recessed part 241C is formed, leaning of piston 223 increases only by the recess amount of recessed part 241C. As a result, support length of the sliding part which supports one side of piston 223 becomes shorter as shown by L2 in FIG. 2.

Therefore, in the configuration shown in FIG. 2, the leaning of piston 223 becomes excessive. Consequently, when periphery 242 of recessed part 241C enters cylindrical-shape hole 216 in the compression stroke, there is the possibility that periphery 242 locally collides with end face 216A of cylindrical-shaped hole 216, and noise increases.

Next, a configuration solving such a problem will be described with reference to FIGS. 4 to 7. FIG. 4 is a cross section showing a state where the piston of the hermetic-type compressor in the first embodiment of the invention is positioned at the bottom dead center. FIG. 5 is a cross section showing a state where the piston of the hermetic-type compressor is positioned at the top dead center. FIG. 6 is a bottom view of the piston of the hermetic-type compressor. FIG. 7 is a cross section of a compression part showing a state where the piston which leans is positioned at the bottom dead center.

As shown in FIGS. 4 and 5, the hermetic-type compressor has sealed container 301, electric mechanism 304, and compression mechanism 305. Lubricant oil 306 is stored at the bottom of sealed container 301. Electric mechanism 304 has stator 302 and rotor 303 and is disposed in sealed container 301. Compression mechanism 305 is also disposed in sealed container 301 and driven by electric mechanism 304.

Concretely, compression mechanism 305 has shaft 310, cylinder block 314, piston 423, and connecting rod 326. Shaft 310 has main shaft part 311 rotated by electric mechanism 304 and eccentric shaft part 312 formed eccentrically at one end of main shaft part 311. Main shaft part 311 is fixed to the shaft center of rotor 303.

Oil support path 313 is provided on the inside and the peripheral face of shaft 310, and one end of oil support path 313 extends in the axial direction in eccentric shaft part 312. Oil support path 313 is communicated with an oil support path (not shown) which is open at the upper end of eccentric shaft part 312. A branch oil path (not shown) which is branched from oil supply path 313 in a radius direction and is open is provided in a some midpoint of eccentric shaft part 312. The lower end of main shaft part 311 extends so that the other end of oil supply path 313 is dipped in predetermined depth in lubricant oil 306.

Cylinder block 314 has an almost cylindrical shaped cylindrical hole 316 constructing compression chamber 315 and bearing 320 rotatably supporting main shaft part 311. Cylindrical hole 316 and bearing 320 are disposed so as to be fixed in predetermined positions. Cylindrical hole 316 and bearing 320 are disposed so that their axes are orthogonal to each other. Bearing 320 serves as a cantilever bearing by axially supporting the end on the side of eccentric shaft part 312 in main shaft part 311 of shaft 310. In cylinder block 314, notch 319 is formed in an upper wall on which lubricant oil 306 falls, in the peripheral wall of cylindrical hole 316.

Piston 423 is reciprocatably inserted in cylindrical hole 316 and has sliding face 423C which slides on the inner wall of cylindrical hole 316 as shown in FIG. 6. The reciprocation direction of piston 423 is substantially the horizontal direction. By connecting rod 326, eccentric shaft part 312 and piston 423 are connected to each other. Specifically, one end of connecting rod 326 is coupled to eccentric shaft part 312, and the other end is coupled to piston 423 via piston pin 425 inserted in piston pin hole 423 as shown in FIG. 6. Connecting rod 326 and piston 423 construct piston assembly 440.

In piston 423, piston pin hole 423A is formed in a direction orthogonal to the axis of piston 423. Compression mechanism 305 has piston pin 425 inserted in piston pin hole 423A. Connecting rod 326 is coupled to piston pin 425 so as to be rotatable about the axis of piston pin 425.

Next, cylindrical hole 316 and piston 423 will be described in detail with reference to FIGS. 6 and 7. As shown in FIG. 7, the dimension in the axial direction of cylindrical hole 316 is set so that, when piston 423 is positioned in the bottom dead center, the end on connecting rod 326 side of piston 423 protrudes from end face 316A on the side of shaft 310 of cylindrical hole 316.

The inner face of cylindrical hole 316 is constructed by, as shown in FIG. 7, straight part 318 in which the inside diameter dimension is constant in the axial direction only in an interval of predetermined length L from the top dead center side and tapered part 317 whose inside diameter dimension increases from Dt to Db (>Dt) toward the bottom dead center. That is, cylindrical hole 316 has tapered part 317 whose inside diameter dimension gradually increases in a direction in which piston 423 moves from the top dead center to the bottom dead center. Cylindrical hole 316 has end face 316A as the end on the side of shaft 310.

The border between straight part 318 and tapered part 317 is the start point of tapered part 317, and is inflection part 317A at which the change rate of taper angle is large.

As shown in FIGS. 6 and 7, the outside diameter of piston 423 is the same in full length. That is, piston 423 does not have a tapered shape. In the outer peripheral face (sliding face 423C) of piston 423, a plurality of recessed parts 441A, 441B, 4411C, and 4412C are provided. Recessed parts 441A and 441B close to compression chamber 315 are formed in an annular shape extending in the entire outer periphery of piston 423. The space volume of each of recessed parts 441A and 441B is 6 mm³ and the interval between them is set to 2 mm.

Recessed parts 4411C and 4412C furthest from compression chamber 315 do not have an annular shape. Recessed parts 4411C and 4412C are formed to, mainly, reduce the area of contact with cylindrical hole 316 of piston 423 and hold lubricant oil 306. When recessed parts 4411C and 4412C hold lubricant oil 306, the sliding face with cylindrical hole 316 of piston 423 can be made lubricant. Therefore, when it is necessary to further reduce the weight of piston 423, recessed parts 4411C and 4412C may be formed deeper or wider.

In FIG. 6, recessed part 4412C is shown representatively. Recessed part 4411C has a similar shape. The outline of recessed part 4412C extends so that its width is gradually decreased from the part parallel to recessed parts 441A and 441B toward end 423B side on connecting rod 326 side, and the terminating end extends oppositely toward compression chamber 315 side.

As shown in FIG. 6, recessed parts 4411C and 4412C are formed symmetrically with respect to axis X as the center of piston pin hole 423A, and their terminating end extends to piston pin hole 423A. Therefore, recessed parts 4411C and 4412C are provided so as to surround piston pin hole 423A, and extension part 423D which extends to the inside of recessed parts 4411C and 4412C is formed in end part 423B. Extension part 423D serves as a part of end part 423B of piston 423. Recessed parts 4411C and 4412C are formed so as to be recessed to the inside in a radial direction of piston 423 and hold lubricant oil 306.

The volume of space formed by the inner face (straight part 318) of cylindrical hole 316 of recessed parts 4411C and 4412C is set to 6 mm³ or larger. Since recessed parts 4411C and 4412C do not face straight part 318, an imaginary state is assumed. An interval of 1.5 mm (interval including the dimension of periphery 442 which will be described later) is provided for recessed part 441B using the deepest point of recessed parts 4411C and 4412C as a base point. The volume of recessed parts 4411C and 4412C can be set arbitrarily as described above.

Recessed parts 4411C and 4412C are provided so as to surround piston pin hole 423A and are therefore communicated with piston pin hole 423A. Specifically, recessed parts 4411C and 4412C are first and second recessed parts formed in positions symmetrical with respect to axis X of piston 423 passing through the center of piston pin hole 423A. Recessed parts 4411C and 4412C communicate with each other via piston pin hole 423A.

Further, a section corner of periphery 442 of recessed parts 4411C and 4412C is formed in an inclined face of about 30°.

Recessed parts 4411C and 4412C are provided in positions symmetrical with respect to axis X as a center in the surface of piston 423. In this case, it is unnecessary to provide recessed part 4411C with extension part 423D. However, by employing the same shape, it becomes unnecessary to recognize the side in the vertical direction of piston 423 at the time of assembly, and workability improves.

In the configuration, piston 423 serves as a component of piston assembly 440 by making piston pin 425 inserted in piston pin hole 423A penetrate connecting rod 326, and is assembled as compression mechanism 305. In this case, extension part 423D is disposed so as to be the bottom face as shown in FIG. 7.

As shown in FIG. 7, in a state where piston 423 is positioned at the bottom dead center, extension part 423D faces (comes into contact with) the corner of end face 316A of cylindrical hole 316. The dimensional relations between piston 423 and end face 316A on the side of connecting rod 326 in cylindrical hole 316 are set to achieve such a state. Specifically, when piston 423 is positioned at the bottom dead center, the lower part in the vertical direction of piston 423 which comes into contact with end face 316A as an end on the side of shaft 310 of cylindrical hole 316 is extension part 423D as a part of sliding face 423C.

The operation of the hermetic-type compressor constructed as mentioned above will now be described. By applying current to electric mechanism 304, rotor 303 of electric mechanism 304 rotates shaft 310, rotary motion of eccentric shaft part 312 is converted to reciprocating motion via connecting rod 326 and the reciprocating motion is transmitted to piston 423. Consequently, piston 423 inserted in cylindrical hole 316 (compression chamber 315) of cylinder block 314 reciprocates in cylindrical hole 316. By the reciprocating motion of piston 423, refrigerant gas from a cooling system (not shown) is taken into compression chamber 315 and compressed. After that the gas is discharged again to the cooling system.

The lower end part of oil supply path 313 functions as a pump using centrifugal force by rotation of shaft 310. By the pumping action, lubricant oil 306 at the bottom of sealed container 301 passes through oil supply path 313, is pumped up, and jets and scatters to respective directions from the oil supply path and the branched oil path provided for eccentric shaft part 312.

Lubricant oil 306 jetted from the oil supply path collides with the ceiling face of sealing container 301 and scatters to mainly cool compression mechanism 305 and make the sliding part lubricant. Lubricant oil 306 from the branched oil path flies almost horizontally in all circumferences in sealed container 301, is supplied mainly to piston pin 325, piston 423, and the like, and makes the sliding part lubricant.

In the reciprocating motion of piston 323, in the beginning of a compression stroke (around the bottom dead center), blowby gas is hardly generated, and sliding resistance of piston 423 is small. Just before piston 423 reaches a position around the top dead center, the pressure in compression chamber 315 further increases. Since the gap between sliding face 423C of piston 423 and tapered part 317 becomes small at the top dead center side, occurrence of blowby gas can be reduced.

In other words, in a state where piston 423 is positioned in the bottom dead center, lubricant oil 306 is supplied amply from notch 319 formed in the upper wall of cylindrical hole 316 to recessed parts 4411C and 4412C formed in sliding face 423C of piston 423 and is held. A part of lubricant oil 306 is supplied to recessed parts 441A and 441B and held. Consequently, also when piston 423 moves to straight part 318 where the gap is narrow, a larger amount of lubricant oil is supplied to the sliding part formed by piston 423 and straight part 318. Therefore, the lubricant oil makes the sliding part lubricant and sealed. As a result, occurrence of gas leakage is prevented, and the volumetric efficiency can be improved.

Further, preferably, cylindrical hole 316 has straight part 318 provided on the top dead center side of piston 423 relative to tapered part 317. With the configuration, the sealed part of piston 423 around the top dead center where the pressure increases most in the compression stroke can be formed in straight part 318 whose inside diameter dimension is constant in the axial direction. In the sealed part, the distance in the axial direction of the minimum gap between piston 423 and cylindrical hole 316 is long, so that action of preventing occurrence of gas leakage accompanying increase in pressure of the refrigerant gas is large. When piston 423 is positioned in tapered part 317 around the bottom dead center, the gap in the radius direction is wide, so that the sliding loss is small. As a result, higher efficiency can be achieved.

In a state where piston 423 is positioned at the bottom dead center, the end part on the side of connecting rod 326 of piston 423 is exposed from the end part on the side of shaft 310 of cylindrical hole 316. Consequently, a large amount of lubricant oil 306 scattered and supplied is adhered to the surface of exposed piston 423 and can be supplied to the sliding part and the sealed part as piston 423 reciprocates. As a result, sliding loss is reduced, and higher efficiency can be achieved together with the above-described prevention of occurrence of gas leakage.

By making periphery 442 of recessed parts 4411C and 4412C an inclined face, the wedge film effect of lubricant oil 306 is obtained, and an oil film can be reliably formed in the gap between piston 423 and cylindrical hole 316.

When piston 423 is positioned at the bottom dead center, the bottom dead center side of piston 423 leans downward in the vertical direction in the gap between cylindrical hole 316 and piston 423. However, extension part 423D is in contact with the corner of end face 316A of cylindrical hole 316. Consequently, piston assembly 440 leans due to its own weight, so that periphery 442 is deviated vertically downward from cylindrical hole 316 and does not collide with the lower corner of end face 316A. Therefore, occurrence of collision noise is suppressed, and reduction in noise can be achieved. Connecting rod 326 is coupled to piston pin 425 so as to be rotatable about the axis of piston pin 425. Consequently, piston 423 does not rotate about the axis, and extension part 423D reliably comes into contact with the corner of end face 316A.

Since recessed parts 4411C and 4412C communicate with piston pin hole 423A, a circulation path is formed by lubricant oil 306 scattered and supplied around the bottom dead center of piston 423, and piston 423 is cooled by lubricant oil 306. By the cooling, the temperature of piston 423 decreases. Accordingly, temperature rise in compression chamber 315 is suppressed, and deterioration in the volume efficiency caused by heat reception is prevented.

Further, in the case of driving an inverter at operation frequency equal to or less than power-supply frequency, by the synergetic effect of maintenance of oil retentivity by the capillary action of recessed parts 441A and 441B, formation of eddying flow by the labyrinth effect, formation of decelerating flow accompanying passage in recessed parts 441A, 441B, 4411C, and 4412C of leakage flow of the refrigerant gas, and the like, leakage of refrigerant can be suppressed.

As a result, particularly, the refrigeration capacity and efficiency at the time of operating the hermetic-type compressor in a low operation frequency range equal to or less than the power-supply frequency can be increased. The effect will be described specifically in a second exemplary embodiment.

Second Exemplary Embodiment

FIG. 8 is an enlarged cross section of a compression part showing a state where a piston of a hermetic-type compressor in a second embodiment of the present invention is positioned at the bottom dead center. FIG. 9 is an enlarged cross section of the compression part, showing a state where the piston is positioned at the top dead center. FIG. 10 is a vertical cross section of a piston assembly of the hermetic-type compressor in the second embodiment. FIG. 11 is a top view of the compression part, showing a state where the piston of the hermetic-type compressor in the second embodiment is in a compression stroke. FIG. 12 is a characteristic diagram of the piston lateral pressure load with respect to crank angle of the hermetic-type compressor in the second embodiment.

In the embodiment, a general configuration of a compressor will be described mainly with respect to parts different from the first embodiment by quoting the description (including the reference numerals) of the first embodiment and FIGS. 4 and 5.

The part different from the first embodiment is the configuration of recessed parts formed in a piston, and the other configuration is similar to the first embodiment. Therefore, the piston having the different configuration will be mainly described.

As illustrated in FIGS. 8 and 9, the outside diameter of piston 323 is constant in full length. In the surface of piston 323, three recessed parts 341A, 341B, and 341C are provided at predetermined intervals. Each of recessed parts 341A, 341B, and 341C is formed in an annular shape in the entire circumference in the surface of piston 323.

The volume of space formed by each of recessed part 341A formed in a position closest to compression chamber 315 and recessed part 341B in the second closest position and the inner face (straight part 318) of cylindrical hole 316 is set to 6 mm³. The interval between recessed parts 341A and 341B is set to 2 mm.

The volume of space formed by recessed part 341C in a third position and the inner face (straight part 318) of cylindrical hole 316 is set to 6 mm³ or larger. However, since recessed part 341C does not face straight part 318, an imaginary state is assumed. Between recessed parts 341C and 341B, the interval of 1.5 mm (interval including the dimension of periphery 342 which will be described later) is provided using the deepest point of recessed part 341C as a base point. A part of recessed part 341C is communicated with piston pin hole 323A. Recessed part 341C is formed for purposes similar to those of recessed parts 4411C and 44112C in the first embodiment. Therefore, the volume of recessed part 341C can be arbitrarily set.

As shown in FIG. 8, end part 323B on the side opposite to the compression chamber (on the side of connecting rod 326) of piston 323 positioned at the bottom dead center is exposed only by length A from end face 316A on the shaft side of cylinder block 314. Piston 323 is formed in such dimensions. In other words, the dimension in the axial direction of cylindrical hole 316 is set so that, when piston 323 is positioned at the bottom dead center, the corner of end face 316A of cylindrical hole 316 comes into contact with end part 323B. End part 323B is the outer peripheral face between the end on the side of connecting rod 326 in piston 323 and recessed part 341C having the annular shape.

Further, in cylinder block 314, notch 319 is provided in the upper wall on which lubricant oil 306 falls in the peripheral wall of cylindrical hole 316 in a manner similar to the first embodiment. By notch 319, at least recessed part 341C is exposed in a state where piston 323 is positioned at the bottom dead center. In other words, recessed part 341C is defined as a part of recesses in the configuration having the plurality of recesses 341A, 341B, and 341C.

As shown in FIG. 8, recessed part 341C is formed so that, in a state where piston 323 reaches the position of the bottom dead center, all of recessed part 341C is positioned on the side of the top dead center only by length B from end face 316A of cylindrical hole 316. End face 323C on the side of compression chamber 315 of piston 323 is positioned on the side of tapered part 317 only by distance of length C. Further, as shown in FIG. 10, periphery 342 of recessed part 341C has a shape inclined at almost 30° in cross section.

FIG. 11 shows disposition of piston 323 when the crank angle is 320 degrees in the compression stroke. As shown in FIG. 12, the crank angle of 320 degrees is the angle at which the lateral pressure load of piston 320 becomes the maximum. The maximum lateral pressure load acts on the lateral pressure load sliding part on the side face in the horizontal direction of cylindrical hole 316. At this time, inflection part 317A between straight part 318 and tapered part 317 is positioned in the range of the width of recessed part 341C in piston 323. In FIG. 11, to clearly show that inflection part 317A is positioned in the range of the width of recessed part 341C, the clearance between piston 323 and straight part 318 of cylindrical hole 316 is illustrated largely.

The operation of the hermetic-type compressor constructed as mentioned above will now be described. By applying current to electric mechanism 304, rotor 303 of electric mechanism 304 rotates shaft 310, rotary motion of eccentric shaft part 312 is converted to reciprocating motion via connecting rod 326, and the reciprocating motion is transmitted to piston 323. By the motion, piston 323 reciprocates in cylindrical hole 316.

Piston 323 shifts from the bottom dead center position shown in FIG. 8 to the compression stroke of compressing the refrigerant gas. In a compression initial state during the shift to the top dead center side shown in FIG. 9, rise of pressure in compression chamber 315 is small. Consequently, even if the clearance between tapered part 317 formed in cylindrical hole 316 and the sliding face (peripheral face) of piston 323 is relatively large, blowby gas is hardly generated by the sealing effect of the lubricant oil. Since the clearance is large, sliding resistance of piston 323 is also small.

When the compression stroke progresses and the crank angle becomes 320 degrees, piston 323 is in the position shown in FIG. 11. At this time, the lateral pressure load of piston 323 becomes the maximum value as shown in FIG. 12.

In the configuration shown in FIG. 3 described in the first embodiment, when the lateral pressure load becomes the maximum, the surface pressure of a sliding part in the side face of piston 223 locally rises and inflection part 217A as the start point of tapered part 217 easily slides on the sliding part. As a result, a lubricant state deteriorates, and there is the possibility such that sliding sound increases.

In the second embodiment, however, inflection part 317A having high change rate of the taper angle as the start point of tapered part 317 is positioned in the range of the width of recessed part 341C in piston 323. In addition, since the depth of recessed part 341C is assured, inflection part 317A is apart from the bottom of recessed part 341C in a state where inflection part 317A faces recessed part 341C. Therefore, even when the lateral pressure load increases, the lubricant state does not decrease in inflection part 317A in which an oil film is not easily formed, inflection part 317A does not locally slide, and no sliding sound is generated.

When the compression stroke further progresses, the pressure of the refrigerant gas in compression chamber 315 gradually increases. Just before piston 323 reaches a position near the top dead center shown in FIG. 9, the pressure in compression chamber 315 further rises. On the top dead center side, the gap between the sliding face of piston 323 and tapered part 317 becomes small, so that generation of blowby gas can be reduced. At this time, straight part 318 formed in cylindrical hole 316 reduces leakage of the refrigerant gas increased to predetermined discharge pressure more than tapered part 317.

In the state where piston 323 is positioned at the bottom dead center, the side of connecting rod 326 of piston 323 is exposed from cylinder block 314. Lubricant oil 306 scattered from the upper end of shaft 310 is amply supplied from notch 319 formed in the upper wall of cylindrical hole 316 to recessed part 341C formed in the sliding face of piston 323 and is held. A part of lubricant oil 306 is supplied to recessed parts 341A and 341B. Consequently, the lubricant oil supplied to the gap between the inner peripheral face of cylindrical hole 316 of cylinder block 314 and the sliding face of piston 323 becomes large in the compression stroke.

During movement of piston 323 to the top dead center, all of piston 323 is positioned in cylindrical hole 316. Due to this, escape of lubricant oil 306 held in recessed parts 341A, 341B, and 341C from cylindrical hole 316 is suppressed. In addition, lubricant oil 306 is easily carried to straight part 318 in which sliding resistance is highest.

Further, end face 323C on the side of the compression chamber of piston 323 is positioned on the side of tapered part 317 only by distance of length C in FIG. 8 at the bottom dead center. Consequently, when piston 323 moves from the bottom dead center to the top dead center in the compression stroke, a part of lubricant oil 306 adhered to the surface of piston 323 moves to the top dead center side, and a part of lubricant oil 306 adhered to the surface of cylindrical hole 316 is also taken and supplied to the gap between piston 323 and cylindrical hole 316 as piston 323 moves.

In the state shown in FIG. 8, the end face on the side of compression chamber 315 of piston 323 is positioned in tapered part 317. Consequently, the gap between piston 323 and cylindrical hole 316 is larger than that in the case where piston 323 is positioned in straight part 318. The amount of lubricant oil 306 held in the space of the gap is accordingly larger.

Therefore, also when piston 323 moves to straight part 318 in which the gap is narrow, a larger amount of lubricant oil is supplied to the sliding part formed by piston 323 and straight part 318, and the sliding part can be made lubricant and sealed. As a result, occurrence of gas leakage is prevented, and volume efficiency can be improved. The configuration can be applied also to the first embodiment.

Since recessed part 341C is provided in the annular shape in the sliding face of piston 323, for example, by widening the width of recessed part 341C in the axial direction of piston 323, the area of recessed part 341C can be maximized.

With the configuration as described above, the sliding area between cylindrical hole 316 (compression chamber 315) and piston 323 is reduced maximally, and sliding resistance can be decreased. In addition, lubricant oil 306 can be supplied uniformly and stably to the lubricant part and the sealing part in the entire circumference of piston 323. Consequently, poor lubrication and deterioration in sealing performance caused by nonuniform and unstable oil supply can be prevented.

Further, periphery 342 of recessed part 341C is constructed as a face inclined from the surface in the axial direction of piston 323 by about 30° in a sectional shape. Consequently, when piston 323 reciprocates, lubricant oil 306 held in recessed part 341C gains force in recessed part 341C. Along the inclination of periphery 342 of recessed part 341C, lubricant oil 306 is pulled in the gap between piston 323 and cylindrical hole 316, enters the gap, and acts so as to correct the inclination of piston 323. In such a manner, a so-called wedge film effect is produced in the gap between piston 323 and cylindrical hole 316.

As a result, by the wedge film effect of lubricant oil 306, the inclination of piston 323 is corrected so as to be reduced, and the gap with cylindrical hole 316 in the entire circumference of piston 323 is made uniform. Therefore, lubricating oil 306 is carried more easily to the sliding part and the sealing part around the top dead center in which the gap is particularly formed narrow, and the frequency of inevitable local metal contact can be reduced.

The angle of periphery 342 of recessed part 341C is not limited to about 30°. The angle may be any angle at which the wedge film effect such that, as described above, when piston 323 reciprocates, lubricant oil 306 held in recessed part 341C is pulled in the gap between piston 323 and cylindrical hole 316 is easily produced. That is, it is sufficient to the angle properly in accordance with the reciprocation speed of piston 323 and the like. In the embodiment, the angle with respect to the surface in the axial direction of piston 323, of periphery 342 is preferably in the range of 25° to 35°. However, in a sectional shape having an inclination angle of 45° or less or an equivalent curved shape, the angle may be any angle at which lubricant oil 306 held in recessed part 341C is pulled in the gap between piston 323 and cylindrical hole 316.

As a result, a larger amount of lubricant oil 306 can be supplied to the gap between cylindrical block 314 and piston 323, lubricant oil 306 is excellently held, and sealing performance can be improved. Further, with ample supply of lubricant oil 306, the sliding resistance of piston 323 can be reduced, so that the compression efficiency is improved, an input is reduced, and higher efficiency can be achieved. The configuration may be applied to recessed parts 4411C and 4412C of the first embodiment.

Piston assembly 340 has a cantilever configuration that the deadweight of piston assembly 340 is supported only by the sliding part of piston 323 inserted in cylindrical hole 316. Consequently, around the bottom dead center at which piston 323 is exposed from cylindrical hole 316 most, the bottom dead center side of piston 323 leans downward in the vertical direction in the gap between piston 323 and cylindrical hole 316.

However, periphery 342 on the connecting rod side of recessed part 341C is positioned on the top dead center side relative to end face 316A of cylindrical hole 316. End part 323B of piston 323 and the corner of end face 316A of cylindrical hole 316 are in contact with each other. Consequently, periphery 342 of recessed part 341C is not deviated vertically downward from cylindrical hole 316 and does not collide with the lower corner of end face 316A. Therefore, occurrence of collision noise is suppressed, and reduction in noise can be achieved.

A part of recessed part 341C is communicated with piston pin hole 323A. That is, preferably, the upper and lower sides of recessed part 341C communicate with each other via piston pin hole 323A. With the configuration, a circulating path that lubricant oil 306 scattered and supplied to the upper part of piston 323 around the bottom dead center passes through circular shaped recessed part 341C and is ejected downward via the end face of piston pin hole 323A is formed. Piston 323 heated by high-temperature, high-pressure refrigerant gas is cooled by relatively-low-temperature lubricant oil 306 passing through the circulating path. By the cooling, the temperature of piston 323 decreases. Accordingly, temperature rise in compression chamber 315 is suppressed, and deterioration in the volume efficiency caused by heat reception can be prevented.

Further, in the case of driving an inverter at operation frequency equal to or less than power-supply frequency, particularly, in a low-speed operation of 30 r/sec or less, the reciprocating motion speed of piston 323 becomes slow and, in addition, the supply amount of lubricant oil 306 supplied by the pumping action of shaft 310 decreases. Consequently, the amount of lubricating oil 306 sprayed from eccentric shaft part 312 into hermetic container 301 decreases.

However, around the bottom dead center, at least recessed part 341C is exposed from cylindrical hole 316. Consequently, lubricant oil 306 is reserved mainly in recessed part 341C and supplied to the sealing part. The oil retentivity is maintained by the capillary action of recessed parts 341A and 341B, and eddying flow by the labyrinth effect is formed. Further, after leakage flow of the refrigerant gas passes through recessed parts 341A, 341B, and 341C, decelerating flow by contraction flow is formed. By the synergetic effect of formation of eddying flow by the labyrinth effect, formation of decelerating flow by contraction flow, and the like, leakage of refrigerant can be suppressed. As a result, particularly, the refrigeration capacity and efficiency at the time of operating the hermetic-type compressor in a low operation frequency range equal to or less than the power-supply frequency can be increased.

Hereinafter, a result of conducting a confirmatory experiment of coefficient of performance (C.O.P.) of the hermetic-type compressor in the embodiment will be described with reference to FIGS. 13 to 15. The coefficient of performance is the ratio of refrigeration capacity to an applied input and is generally used as an index indicating the efficiency of a compressor. In tests, R600a (isobutane) was used as the refrigerant. The operation frequency was 27 r/sec, and operation conditions close to operation conditions in a refrigerator were evaporation temperature of −30° C. and condensation temperature of 40° C.

FIG. 13 is a characteristic diagram of coefficient of performance with respect to space volume of recessed parts 341A and 341B. FIG. 14 is a characteristic diagram of the coefficient of performance with respect to distances among neighboring recessed parts 341A, 341B, and 341C. FIG. 15 is a characteristic diagram of the coefficient of performance with respect to operation frequency of the compressor.

In FIG. 13, the vertical axis denotes the coefficient of performance of the compressor, and the horizontal axis denotes the sum of volume of space surrounded by the section of recessed parts 341A and 341B and the extension face of the outside diameter of piston 323.

The test result shown in FIG. 13 is a result of conducting the test using the recessed parts on the side of compression chamber 315 as a plurality of recessed parts 341A and 341B having small sectional area. However, the invention is not limited to a plurality of recessed parts. One recessed part formed in a volume with which the result shown in FIG. 13 is obtained may be employed.

As obvious from FIG. 13, it is preferable to set the space volume of recessed parts 341A and 341B in a range T of 0.25 mm³ to 25 mm³ (inclusive). By such setting, the coefficient of performance higher than that in the case where the space volume is smaller than 0.25 mm³ and that in the case where the space volume is larger than 25 mm³ can be obtained.

Referring now to FIG. 14, the influence of distance S between neighboring recessed parts 341A, 341B, and 341C will be described. In FIG. 14, the vertical axis denotes the coefficient of performance of the compressor, and the horizontal axis denotes distance S between neighboring recessed parts 341A, 341B, and 341C.

As shown in FIG. 14, by setting the distance between neighboring recessed parts 341A, 341B, and 341C to 1 mm or larger, the coefficient of performance (C.O.P) increases. It is assumed that, by setting distance S between neighboring recessed parts 341A, 341B, and 341C to 1 mm or larger, the gap between the surface of piston 323 and cylindrical hole 316 becomes a reducer. Consequently, the flow rate of a mixed fluid of the refrigerant gas and lubricant oil 306 increases so that the pressure of the mixed fluid is reduced. As a result, the leakage amount from the gap between piston 323 and cylindrical hole 316 further decreases. Therefore, by further reducing the amount of leakage to the side opposite to the compression chamber, reduction in the volumetric efficiency is prevented, and the efficiency of the compressor can be increased.

In the embodiment, recessed parts 341A, 341B, and 341C are formed by setting the distance between neighboring recessed parts 341A, 341B, and 341C to 1 mm or larger. Consequently, in addition to the above-described effects, even in the case where oil in any one of recessed parts 341A, 341B, and 341C becomes discontinuous and the sealing performance deteriorates, the sealing performance can be maintained by the other recessed parts.

Next, with reference to FIG. 15, the characteristics of the coefficient of performance when the compressor of the embodiment is assembled in a refrigeration cycle and the operation frequency of the compressor is changed under predetermined operation load conditions (certain conditions) will be described. The vertical axis indicates the coefficient of performance of the compressor, and the horizontal axis indicates the operation frequency at which the piston is driven. For comparison, as a conventional technique, a result of the case where the operation frequency is set in the range of about 20 r/sec to about 45 r/sec in a state where a compressor having specifications (cylinder volume: 10 ml and capability at the time of operation of 27 r/sec: 74 W) equivalent to those of the embodiment is assembled in a similar refrigeration cycle is shown. In the conventional compressor, the cylindrical hole does not have a tapered part, and recessed part 341C is not formed in the piston.

As obvious from FIG. 15, in the case of low operation frequency at which the effect of reducing power consumption is large in a cooling system such as a refrigerator, the coefficient of performance is largely improved as compared with that in the conventional compressor. It is therefore understood that the sealing performance of piston 323 and cylindrical hole 316 improves dramatically, and the leakage amount can be reduced.

Generally, in a low-speed rotation range, refrigeration capacity is small and the ratio of loss of leakage from the gap between piston 323 and cylindrical hole 316 to the refrigeration capacity becomes high, so that the efficiency of the compressor deteriorates. However, in the embodiment, by the stable sealing by lubricant oil 306 and the labyrinth effect, the amount of leakage from the gap between piston 323 and cylindrical hole 316 can be reduced. Consequently, extreme deterioration in the efficiency of the compressor accompanying deterioration in volume efficiency can be prevented, and power consumption of the cooling system can be largely reduced.

As described above, in the hermetic-type compressor according to the embodiment, a local contact between piston 323 and cylindrical hole 316 is avoided and, simultaneously, the sliding area is minimized and the sliding loss can be minimized. Moreover, lubricant oil 306 contributing to the performance of sealing between piston 323 and cylindrical hole 316 is stably supplied to the gap between piston 323 and cylindrical hole 316 and can be reliably assured between piston 323 and cylindrical hole 316.

As a result, a metal contact causing abrasion and noise is prevented, reliability is improved and, moreover, occurrence of noise can be reduced. Further, by assurance of the sealing performance accompanying assurance of stability of lubricant oil 306, the volumetric efficiency is increased and, as a result, the efficiency of the compressor can be improved. Therefore, higher efficiency and reliability and prevention of occurrence of noise can be simultaneously achieved, and partly contradictory challenges can be solved.

As described above, according to the first and second embodiments, the full length of cylindrical hole 316 is shortened and the hermetic-type compressor is downsized and, moreover, occurrence of contact noise is prevented, and occurrence of abrasion can be reduced. Consequently, higher efficiency, lower noise, and higher reliability of the hermetic-type compressor can be simultaneously achieved.

INDUSTRIAL APPLICABILITY

According to the present invention, the compression efficiency of the hermetic-type compressor is increased, and sound of collision between the piston and the cylindrical hole can be suppressed. The hermetic-type compressor can be widely applied as a hermetic-type compressor for use in a machine using a refrigeration cycle such as an air conditioner or an automatic vending machine.

REFERENCE MARKS IN THE DRAWINGS

114, 214, 314 cylinder blocks

114A, 214A, 319 notches

116, 216, 316 cylindrical holes

123, 223, 323, 423 pistons

123B, 223B lower end parts

226, 326 connecting rods

141A, 141B, 241A, 241B grooves

141C, 241C, 341A, 341B, 341C, 441A, 441B, 4411C, 4412C recessed parts

216A, 316A, 323C end faces

217, 317 tapered parts

217A, 317A inflection parts

218, 318 straight parts

242, 342, 442 peripheries

301 sealed container

302 stator

303 rotor

304 electric mechanism

305 compression mechanism

306 lubricant oil

310 shaft

311 main shaft part

312 eccentric shaft part

313 oil supply path

315 compression chamber

320 bearing

323A, 423A piston pin holes

323B end part

340, 440 piston assemblies

325, 425 piston pins

423B end part

423C sliding face

423D extension part 

1. A hermetic-type compressor comprising: a sealed container storing lubricant oil at its bottom; an electric mechanism disposed in the sealed container; and a compression mechanism disposed in the sealed container and driven by the electric mechanism, wherein the compression mechanism includes: a shaft having a main shaft part rotated by the electric mechanism and an eccentric shaft part formed in the main shaft part; a cylinder block having a cylindrical hole constructing a compression chamber and a bearing rotatably supporting the main shaft part, in which the cylindrical hole and the bearing are disposed so that axis of the cylindrical hole and that of the bearing are orthogonal to each other; a piston reciprocatably inserted in the cylindrical hole and having a sliding face which slides on an inner wall of the cylindrical hole; and a connecting rod connecting the eccentric shaft part and the piston, the cylindrical hole has a tapered part, whose inside diameter dimension gradually increases from a top dead center of the piston toward a bottom dead center, and an end part on the shaft side, a reciprocation direction of the piston is substantially a horizontal direction, a recessed part, which is recessed to an inside in a radial direction of the piston and holds the lubricating oil, is provided in the sliding face of the piston, and a part on the lower side in the vertical direction of the piston, which comes into contact with the end part on the shaft side of the cylindrical hole when the piston is positioned in the bottom dead center, is a part of the sliding face.
 2. The hermetic-type compressor according to claim 1, wherein the piston has a piston pin hole in a direction orthogonal to axis of the piston, the compression mechanism further includes a piston pin inserted in the piston pin hole, the connecting rod is coupled to the piston pin so as to be rotatable about the axis of the piston pin, and the sliding face has an extension part which extends from an end part on the connecting rod side toward the recessed part and, when the piston is positioned at the bottom dead center, and the extension part comes into contact with the end part on the shaft side of the cylindrical hole.
 3. The hermetic-type compressor according to claim 2, wherein the recessed part is one of a plurality of recessed parts, the plurality of recessed parts include first and second recessed parts formed in positions symmetrical with respect to the axis of the piston extending in the center of the piston pin hole, and the first and second recessed parts communicate with each other via the piston pin hole.
 4. The hermetic-type compressor according to claim 1, wherein the recessed part has an annular shape extending in an outer periphery direction of the piston.
 5. The hermetic-type compressor according to claim 4, wherein the cylindrical hole further includes a straight part provided on the top dead center side of the piston relative to the tapered part and an inflection part as a border between the tapered part and the straight part, and the position of the inflection part and the position of the recessed part are set so that when the piston is in a position where lateral pressure load by the piston is maximum, the inflection part is positioned in a range of width in the axis direction of the piston in the recessed part.
 6. The hermetic-type compressor according to claim 5, wherein depth of the recessed part is set so that the inflection part is apart from bottom of the recessed part when the piston is in the position where lateral pressure load by the piston is maximum and the inflection part is positioned in the range of width in the axial direction of the piston in the recessed part.
 7. The hermetic-type compressor according to claim 4, wherein the recessed part is one of a plurality of recessed parts, the plurality of recessed parts include a first recessed part having an annular shape and a second recessed part having an annular shape positioned on the compression chamber side relative to the first recessed part and extending in the outer periphery direction of the piston, and space volume of the second recessed part is in a range from 0.25 mm³ to 25 mm³ (inclusive).
 8. The hermetic-type compressor according to claim 7, wherein an interval between the first and second recessed parts is 1 mm or larger.
 9. The hermetic-type compressor according to claim 1, wherein the cylindrical hole has a straight part provided on the top dead center side of the piston relative to the tapered part.
 10. The hermetic-type compressor according to claim 9, wherein when the piston is positioned at the bottom dead center, an end face on the compression chamber side of the piston is positioned in the tapered part of the cylindrical hole.
 11. The hermetic-type compressor according to claim 1, wherein a notch for exposing a part of the recessed part when the piston is positioned in the bottom dead center is provided in an upper part of the end part on the shaft side of the cylindrical hole in the cylinder block.
 12. The hermetic-type compressor according to claim 1, wherein a sectional shape of a periphery as the border from the sliding face to the recessed part has an inclination angle of 45° or less with respect to a surface in the axial direction of the piston or an equivalent curved shape. 